1. Field of the Invention
The present invention relates to an object, a method, a manufacturing method, a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor device, a display device, a light-emitting device, a method for driving any of them, or a method for manufacturing any of them. In particular, the present invention relates to, for example, a light-emitting element. In particular, the present invention relates to, for example, a light-emitting element containing an organic compound that is capable of converting triplet excited energy into luminescence. The present invention also relates to, for example, a light-emitting device, an electronic appliance, and a lighting device each of which includes the light-emitting element.
2. Description of the Related Art
In recent years, a light-emitting element using a light-emitting organic compound or inorganic compound as a light-emitting material has been actively developed. In particular, a light-emitting element called an EL (electroluminescence) element has attracted attention as a next-generation flat panel display element because it has a simple structure in which a light-emitting layer containing a light-emitting material is provided between electrodes, and characteristics such as feasibility of being thin, lightweight, and highly responsive to input signals, and able to be driven with direct current at low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since such a light-emitting element is a plane light source, the light-emitting element is considered to be applicable to a light source such as a backlight of a liquid crystal display and lighting.
In the case where the light-emitting substance is an organic compound having a light-emitting property, the emission mechanism of the light-emitting element is a carrier-injection type. That is, by applying voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes recombine to make the light-emitting substance excited, and light is emitted when the excited state returns to a ground state. There are two types of the excited states that are possible: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio of S* to T* in a light-emitting element is thought to be 1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Light emission from the singlet excited state (S*) is referred to as fluorescence where electron transition occurs between the same multiplicities. On the other hand, light emission from the triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different multiplicities. At room temperature, observations of a compound which emits fluorescence (hereinafter referred to as a fluorescent compound) usually show only fluorescence without phosphorescence. Therefore, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
On the other hand, when a phosphorescent compound is used as a light-emitting organic compound, the internal quantum efficiency can be theoretically increased to 100%. In other words, the emission efficiency can be four times as much as that of the fluorescence compound. For this reason, light-emitting elements using a phosphorescent compound have been recently under active development so that high-efficiency light-emitting elements can be achieved.
In particular, an organometallic complex in which iridium or the like is a central metal has attracted attention as a phosphorescent compound owing to its high phosphorescence quantum yield. As a typical phosphorescent material emitting green to blue light, there is a metal complex in which iridium (Ir) is a central metal (hereinafter referred to as an “Ir complex”) (e.g., see Patent Document 1, Patent document 2, and Patent Document 3). Disclosed in Patent Document 1 is an Ir complex where a triazole derivative is a ligand.